The present invention relates to a process for controlling an overcurrent tripping device of a high-speed d.c. circuit-breaker, particularly high-speed d.c. circuit-breakers for power supply systems which feed electric d.c.-operated railway lines. A transformer is provided in the d.c. circuit and a filtering device connected on the output side of the transformer for producing an input voltage from which a signal, dependent on the rate of rise of the current (di/dt) as well as a signal corresponding to any sudden change in current, is derived. A first disconnect signal can be generated from these signals via parallel signal evaluation devices. The first disconnect signal is for the high-speed d.c. circuit-breaker in conjunction with a current-rise-time controller and a second disconnect signal in conjunction with a current-step-change controller.
A process of the above described type has been described by the article "Neues Streckenschutzkonzept gegen Speiseleitungsstorungen bei Gleichstrom-Verkehrsnetzen" ("New pilot-wire protection concept against supply line interference in d.c. transit networks") in the German publication Brown Boveri Mitteilungen (Brown Boveri News), 9/10-83, pp. 372-378. One of the protection devices described therein uses the magnitude of the current step changes .delta.I and, in parallel to this, the current rise as a function of time (di/dt) as a basis for tripping. Despite using this combined measuring criteria for the current, the reliability of the tripping process can prove unsatisfactory if interference exists on the current flowing in the power supply system. Such interference can be due, in particular, to the conventional method of operation of the rectifier plants which feed d.c.-operated railway lines. This is related to the fact that the d.c. current needed to run d.c.-operated railway lines is generated by rectifying the current supplied by a three-phase a.c. system. A ripple is thus superimposed on the d.c. current flowing through the d.c.-operated railway lines. This ripple is also detected by the protection device.
The object of the present invention is to improve the process discussed in the above mentioned article for controlling an overcurrent tripping device such that a ripple superimposed on the d.c. current does not cause erroneous tripping of the high-speed d.c. circuit-breaker.